Susan Domizi asked three questions regarding a rather confusing claim in
some marketing literature. I don't understand the claim either (and I
wonder if the original author did him or herself), but might be able to
throw a little light on the subject ..........
My understanding is that the toxicity of arsenicals depends on the
nature of the compounds, and particularly on the valency of the arsenic
atom itself. Trivalent arsenic (i.e. arsenite) is very toxic, whereas
pentavalent arsenic (i.e. arsenate) is rather more benign. Pentavalent
organic arsenic compounds are not toxic at all.
Seaweeds, like other seafoods, contain considerable quantities of
arsenic. In both brown and red seaweeds most of this arsenic is present
as an arsenosugar of some sort. A very small proportion will be present
as an inorganic salt. All will be in the pentavalent state.
The Food Regulations in NZ recognise the low toxicity of organically
bound pentavalent arsenic, and impose no restrictions on the amount a
seafood can contain. There is a 2 mg/kg regulatory limit on inorganic
arsenic though. As far as we know, all NZ seaweeds are below this limit.
To reply directly to Susan's questions:
1> Does anyone have any information that suggests that arsenic from any
of the macroalgae may contribute to cataracts or blindness in animals or
people ?
No. If it did there should be a much larger number of blind people in
Japan.
2> Does anyone have any information suggesting that macroalgae, other
than kelp, contain arsenate ?
Yes. Since there are inorganic arsenates in seawater it would be
surprising if any macroalgae were entirely free of them. However, what
would be more interesting to know is if any of the macroalgae actually
concentrate them. There is some indication that the Japanese phaeophyte
Hizikia fusiformis might do this - the reported levels of inorganic
arsenic it contains certainly wouldn't permit its importation to this
country for human consumption.
3> Does anyone have any suggestions for analytical procedures or other
approaches to validate, repudiate or understand the claims ?
Have a look at the following paper for an introduction to analytical
methods:
Yasui et al (1978) Selective determination of inorganic arsenic (III),
(V), and organic arsenic in biological materials by solvent extraction
atomic absorption spectrophotometry. Agric. Biol. Chem. 42, 2139 - 2145
Hope this helps,
Graeme Bremner
Ministry of Fisheries
Private Bag 1926
Dunedin
New Zealand